That’s because research on cetacean genitalia (dolphins and whales are both cetaceans) is pretty biased towards penis-havers. Comparatively less work has been done studying cetacean vagina-bits. So, a team of scientists decided to break down the cetatriarchy in order to really understand these poorly-explored parts.

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As it turns out, cetacean vaginas have unusual folds with unknown functions that differ between species. In order to really get deep into what’s going on here, the researchers collected 59 cetacean reproductive tracts from twenty species of cetaceans that had washed ashore and died of natural causes. They sliced the parts up, and measured and counted the vaginal folds.

The researchers found lots of interesting tidbits about the porpoise parts: the average vaginal length was 21 centimeters in their specimens, though the length scaled with the size of the animal. This might not sound surprising, but it actually hadn’t been established—genital size is far more often studied in male cetaceans and male mammals in general.

Let me repeat this last point. There wasn’t enough data to compare cetacean vaginal length, because scientists spend all their time measuring penises instead.

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There’s actually a reason for that, though.

“We know a lot less about vaginal morphology in general than we know about penile morphology,” said Diane Kelly, assistant research professor at the University of Massachusetts, Amherst who used to blog about sex for Gizmodo. “Part of it is that penises are easy to get to. They’re out in front,” she said. “Vaginas are more complex. They’re 3D tubes with complex wall anatomy. For a long time there was this assumption that vaginas are simple—but that’s clearly not true.”

Here comes a picture of a bisected dolphin reproductive tract, because this is science and sometimes science looks like a bisected dolphin reproductive tract.

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Image: Orbach et al

Incredible!!!

Like other mammalian vaginas, cetacean vaginas turn out to be very complex. Almost all of the specimens only had folds in the inner half of the vagina, but the number of folds varied between species, and the fold length didn’t scale uniformly with increasing body length.

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We’re not quite sure what the folds are for, but one hypothesis was that they serve to stop water from getting into the part of the vagina where the semen goes. But other mammals that mate in water, like sea otters, don’t have these vaginal folds. The folds might also be muscles shaped to restrict which mates’ sperm make it into the upper reproductive tract, according to the study published today in the journal PLOS One.

“Ducks have these complex twisted vaginal tracts that prevent males they’re not interested in mating with or interested in fathering their offspring from getting their sperm into them,” said Kelly. “Maybe something similar is happening with these cetaceans.”

While this paper was only published today, researchers are already working on the next step, trying to understand how cetacean penises and vaginas fit together. Kelly has even begun working with researchers at the penis end of the cetacean puzzle. She covered some of that work for Gizmodo back in 2015, where you can learn even more about dolphin vaginas.

Ultimately, there is a whole lot we don’t know about cetacean vaginas. So we’re just going to have to cut open some more dead porpoise genitals, do some more studies, and hopefully, watch some whales fuck.